It is known to make use of a look-up table (LUT), after a picture picked up by a camera or the like is digitized, in order to subject the digitized pixel data to such operations as binary coding, density conversion or picture reversal. If a look-up table LUT is used for a binary coding processing, for example, as shown in FIG. 5, 5.times.5 items of pixel data x.sub.ij which exist prior to processing are binary coded by the look-up table LUT to be converted into 5.times.5 items of binary coded pixel data f(x.sub.ij) In the example shown in FIG. 5, the arrangement is such that a threshold value is set to "2", with data of a value equal to or less than "2" being converted to "0" and data of a value greater than "2" being converted to "1".
FIG. 6(a) illustrates the construction of a look-up table for processing 5.times.5 items of pixel data. In FIG. 6(a), the look-up table LUT is composed of a table TBL.sub.0 in which operators to be applied to the pixel data are stored upon being correlated with the individual items of pixel data x.sub.ij, and an arithmetic section 20 for applying operators f.sub.ij outputted from the table TBL.sub.0 to the corresponding items of pixel data x.sub.ij. By way of example, five address lines A.sub.00 -A.sub.04 are connected to the input of the table TBL.sub.0, whereby a maximum of 32 table address spaces can be accessed.
FIG. 6(b) illustrates an example of the configuration of the memory within the look-up table LUT. Each address of the table TBL.sub.0 corresponds to a location of an item of the 5.times.5 pixel data x.sub.ij, as will be understood from the memory configuration. More specifically, an item of pixel data x.sub.00 shown in FIG. 5 corresponds to an address ADDR.sub.00, and an address x.sub.44 corresponds to an address ADDR.sub.24. An operator f.sub.00 to be applied to the pixel data x.sub.00 is stored at the location of address ADDR.sub.00 of the table TBL.sub.0, an operator f.sub.01 to be applied to the pixel data x.sub.01 is stored at the location of address ADDR.sub.01, and an operator f.sub.44 to be applied to the pixel data x.sub.44 is stored at the location of address ADDR.sub.24. The table TBL.sub.0 comprises a RAM. Accordingly, the operators f.sub.ij to be applied to the individual items of pixel data x.sub.ij are stored in a form capable of being altered. In FIG. 6(a), a chip select terminal cs of the table TBL.sub.0 or RAM is held at zero potential, so that the table TBL.sub.0 is being selected at all times.
Data lines D.sub.0 -D.sub.7 for the pixel data x.sub.ij and the output of the table TBL.sub.0 are applied to the arithmetic section 20. The data lines carry eight bits and are capable of expressing data indicative of 256 levels of tones. The arithmetic section 20 applies the operators f.sub.ij from the table TBL.sub.0 to the pixel data x.sub.ij, whereby results f.sub.ij (x.sub.ij) are obtained. With respect to the pixel data x.sub.01, for example, the address ADDR.sub.01 is addressed, the operator f.sub.01 is outputted from the table TBL.sub.0, and the operator f.sub.01 is applied to the pixel data x.sub.01 in the arithmetic section 20.
When such pixel data processing is performed, there are cases where it is desired to apply window processing, i.e. mask processing, to specific pixel data. FIG. 7 illustrates a conventional picture processing apparatus which executes window processing at the same time as pixel data processing. Window processing refers to processing in which the processing of pixel data x.sub.ij is masked and changed over by a window data value. This is performed by providing window data W.sub.ij correlated with the number of items of pixel data x.sub.ij. When an item of window data W.sub.ij is "0", certain processing is applied to the item of pixel data x.sub.ij corresponding thereto (e.g. no processing whatsoever may be applied), and when the item of window data W.sub.ij is "1", processing different from that mentioned above is applied to the item of pixel data x.sub.ij corresponding thereto.
The picture processing apparatus mentioned above is provided with look-up tables LUT.sub.1 and LUT.sub.2 that differ from each other. The look-up tables execute separate processing operations with regard to all of the pixel data x.sub.ij, input the results of processing G.sub.ij (x.sub.ij), Fij(x.sub.ij) to a window processing circuit 21, outputs G.sub.ij (x.sub.ij) as P.sub.ij (x.sub.ij) when the corresponding window data W.sub.ij is "0", and outputs F.sub.ij (x.sub.ij) as P.sub.ij (x.sub.ij) when the corresponding window data W.sub.ij is "1". In the example of FIG. 7, non-conversion operators G.sub.ij are stored in LUT.sub.1, and the outputs G.sub.ij (x.sub.ij) of LUT.sub.1 are output as x.sub.ij. More specifically, as for the output P.sub.ij (x.sub.ij) of window processing circuit 21, the pixel data which prevailed prior to processing are output as is at addresses where the window data is "0", and pixel data which have been subjected to binary coding processing by LUT.sub.2 are output at addresses where the window data is "1".
In this conventional apparatus, hard-wired logic shown in FIG. 8 is used as the window processing circuit 21 With this hard-wired logic, the outputs G.sub.ij (x.sub.ij) and F.sub.ij (x.sub.ij) of the look-up tables LUT.sub.1 and LUT.sub.2 are applied to one input terminal of the AND circuits 30 and 31, respectively, a signal obtained by inverting the window data W.sub.ij by means of an inverter 32 is applied to the other input terminal of the AND circuit 30, and the window data W.sub.ij is applied as is to the other input terminal of the AND circuit 31. Accordingly, the outputs G.sub.ij (x.sub.ij) and F.sub.ij (x.sub.ij) are gated by the window data W.sub.ij. The outputs of the AND circuits 30 and 31 are applied to an OR circuit 33 to obtain the window-processed output P.sub.ij (x.sub.ij).
However, this hard-wired logic is required to be provided for each of the pixels of pixel data to be processed. In the example of FIG. 7, the hard-wired logic is required to be provided for each of 25 pixels. The logic therefore occupies a considerably large surface area of a printed circuit board and entails high cost. Furthermore, considerable time is needed for design, and design modification cannot be easily carried out.
The present invention has been devised to solve the foregoing problems and its object is to provide a picture processing apparatus which reduces hardware costs, enables the processing applied to data to be set very simply and has look-up tables which can be readily modified.